A printed circuit board (PCB) mechanically supports and electrically connects electronic components using conductive traces, pads and other features etched from conductive sheets, such as copper, laminated onto a non-conductive substrate, referred to as PCB laminate or pre-preg. PCBs can be single sided (one copper layer), double sided (two copper layers) or multi-layered. Conductors on different layers are connected with plated through holes called vias. The through holes are formed after all layers of the PCB are laminated together. A drill or laser is commonly used to form each through hole. The through hole walls for multi-layers PCBs can be made conductive and then electroplated with copper to form the plated-through holes.
A conventional fabrication technique for making a multi-layer PCB is to start with a two-sided copper-clad laminate, etch the circuitry on both copper sides, then laminate to the top and bottom another layer of pre-preg and copper foil. Lamination is done by placing the stack of materials in a press and applying pressure and heat for a period of time. This results in a one piece product. Additional layers can be similarly added to the top and/or bottom. Once all the layers are added, the PCB is drilled to form through holes, plated to form plated through holes, and the top and bottom copper foils are etched to form conductive traces on the top and bottom layers of the PCB.
Advanced PCBs may contain components, such as capacitors, resistors or active devices, embedded in the PCB laminate. To embed such a component in the PCB, a cavity is formed in an already cured layer of C stage pre-preg. An adhesive is applied to a bottom surface of the component and the component is placed into the cavity with the bottom side adhesive contacting a bottom surface of the cavity. In most cases the cavity is formed through an entire thickness of the pre-preg so that the surface to which the component is adhered is an underlying copper layer onto which the pre-preg was laminated. Additional layers of the PCB are then added over the component and corresponding pre-preg layer. A problem with this approach is that the adhesive applied to the bottom surface of the component does not have the same thermal properties, such as the coefficient of thermal expansion (CTE), as the surface of the copper to which the component is adhered. As such, during subsequent lamination cycle for adding additional layers to the PCB, the component will move slightly compared to the rest of the PCB stack due to the CTE mismatch. This effects tolerances corresponding to interconnects coupled to contact pads on the top surface of the component. Additionally, the cavity needs to be formed with ample room to allow placement of the component within the cavity. As such, after placement of the component in the cavity there is a gap between the component and the cavity side walls. To secure the component in place, an additional step is performed of adding a filler material, such as A stage pre-preg, into the gap. In practice, the gap may not be completely filled due to voids or bubbles.